I'm still having difficulty believing this could happen.
This morning I brought up groups.io and noticed that I was logged in as someone else - someone who is in a group that I am a member of.
I removed the groups.io cookie, and things went back to normal.
Nobody has been on my computer but me. I think a groups.io cookie got deposited or altered on my computer with someone else's identity.
I'm on Chrome on Windows 10. Really, I was not dreaming.